I do not know the size of screw required but asking a Rolex-accredited repairer might be a good idea. Although even Rolex-accredited repairers can no longer service/repair Oysterquartzes they might well still know the size of screw needed.
(Yes, I know Hamilton & Inches is a Rolex AD but is, presumably, not a specialist Rolex repairer).
Also: I have an OQ and changed the battery recently. Luckily I avoided pinging one of the battery holder screws off into infinity but I did wonder what I'd do if I lost one of the screws. I mooted the possibility of buying a job lot of 'watch screws' and going through them until I found one that would fit.